Study of SiC–nickel alloy bonding for high temperature applications

We have studied the formation of metal/ceramic joints by solid state bonding technique for applications at temperatures >600 °C. The bonding is obtained between silicon carbide (SiC) and Ni-based super-alloy (HAYNES® 214™) via metallic foils (Ni, Ag). In some cases a thin coating on the ceramic or the alloy by the electroless JetMétal™ process has been used. Often used in brazing, nickel, when added to silicon carbide, usually give silicides. These reactions yield the “Pest Effect” (“pesting”) that induces a catastrophic brittleness of this type of assembling. To minimize the reaction of these metals with silicon carbide, addition of elements limiting the “Pest Effect” on the one hand and, diffusion barriers on the other hand, have been performed. Indeed, the choice of the thin Ni0.93 B0.07 coating is based on the ability of boron of improving the mechanical properties of silicides, thus avoiding the “Pest Effect”. However, we demonstrate that boron does not allow one to suppress the joint brittleness. Another new joining method employing a thin Ag coating or a Ag foil was tested. This process revealed the absence of chemical reaction at the Ag/SiC interface, thus proving the beneficial role of silver, which acts as an effective diffusion barrier for nickel beyond a certain thickness. This method has led to fabrication of joints presenting high shear resistance (>40 MPa).